The present invention relates to an eyeglass lens, and, in particular, to an improved shape of a refractive surface of an eyeglass lens.
Conventionally, a refractive surface (hereinafter the front refractive surface) on the front side of an eyeglass lens for use in correcting myopia.
This lens is spherically shaped to facilitate manufacturing and this lens is known as a spherical lens. Generally, a refractive power of a lens is expressed in diopter D, a surface refractive power at the surface of the lens is defined by a curvature .rho. of the surface expressed in m.sup.-1 and a refractive index n of a lens material.
The surface refractive power may be expressed as: EQU (n-1) x .rho.
The surface refractive power of the lens front refractive surface is known as the base curve. A curvature corresponding to the base curve is the curvature of the base curve. The power of the lens is primarily determined by the refractive powers of both its front and back refractive surfaces. Therefore, the base curve can have various values according to the combination of these refractive powers. However, in order to reduce an astigmatism, one of the optical efficiencies of the lens, affecting an eye when a person looks at something through the lens periphery remote from the lens' optical axis, the base curve is practically limited within a certain range of values dependent on the power of the lens.
By way of example, reference is made to FIG. 2, illustrating the characteristic of a lens having a refractive index of 1.50 as one of the astigmatisms. The astigmatism generation state is shown at the rectangular co-ordinates of the graph in which the vertical axis is the refractive power of the base curve and the horizontal axis is the refractive power of the lens when a peripheral portion of the lens is looked through at an angle of 30.degree. relative to the optical axis. The solid line designates the astigmatism generated when looking at a distant object and the figure accompanied with the line indicates the amount of the astigmatism. The solid lines for the astigmatism of 0.3 D are formed on opposite sides of the line indicating non-astigmatism (O D). The broken line similarly designates the astigmatism generated when looking at a close object (30 cm). FIG. 2 clearly shows that the most suitable base curves, having a zero astigmatism, are different for viewing distant objects and close objects. Therefore, base curves designed to fall within the hatched area are usually adopted so as to provide equally good viewing for both distant objects and close objects.
These lenses have been satisfactory for correcting myopia. However, they suffer from the disadvantage that the edge thickness of the lens, i.e. the thickness of the lens at its outer periphery, becomes thicker as the myopia becomes stronger. By way of example reference is made to FIG. 3 in which one example of such a lens, generally indicated as 10, is illustrated. Lens 10 has a power of 6 D and a diameter of 75 mm. Lens 10 is an ordinary plastic lens having a refractive index of 1.5, a base curve of 2.0 D and a center thickness of 2 mm. In this case, the thickness of edge 12 of the lens is 11.7 mm. Therefore, when being used for eyeglasses, it becomes unsightly due to its thick edge.
One known method for solving this problem is to reduce the base curve. Such a lens, generally indicated as 20, is shown in FIG. 4. The base curve of lens 20 is the same as lens 10, however the base curve is reduced to 1.0 D. The thickness of an edge 22 of lens 20 becomes 11.2 mm, which is 0.5 mm thinner than edge 12 of lens 10. However, since the base curve should be determined based upon the optical efficiency as described above, the optical efficiency severely degrades utilizing a base curve of 1.0.
As shown in the graphs of FIGS. 5 and 6, the astigmatism in the fields of vision utilizing lens 10 and lens 20, respectively, differ a great deal, but each degrades rapidly. The vertical axis of the rectangular co-ordinates represents an angle of the field of vision in degrees and the horizontal axis thereof represents an astigmatism in diopters based on a reference refractive power in a sagittal direction of the lens. FIGS. 5 and 6 show the astigmatism in each field of vision for viewing objects infinitely far away (.infin.), 1 m away and 0.3 m away respectively.
Several methods are known for eliminating the unappealing external appearance of the myopia corrective lens. Such methods include making either the front refractive surface and/or the back refractive surface aspherical. One method combines two or more spherical surfaces. These methods have been satisfactory. However, these methods suffer from the problems below.
Making a front refractive surface aspherical is known from Japanese Laid Open Patent Publication No. 94947/78 and Japanese Patent Publication No. 41164/84 (U.S. Pat. No. 4,279,480). In the lens disclosed in Japanese Laid Open Patent Publication No. 94947/78 there is provided a front refractive surface comprising a central portion having a 40 mm diameter formed by one spherical surface and an outer peripheral portion formed by a torus surface having a larger curvature than that of the central portion's spherical surface. However, since there should not be an extremely large difference between curvature of the central portion and the curvature of the outer peripheral portion to prevent the optical efficiency of the outer peripheral portion from being degraded by the large spherical surface central portion, it becomes impossible to make the lens thinner. The lens disclosed in Japanese Patent Publication No. 41164/84 (U.S. Pat. No. 4,279,480) has a front refracting surface formed as an aspherical surface defined by a specific function. When viewed from the center of rotation to the periphery, a refracting surface of the lens first projects forward and then projects backward. The problem associated with this lens lies in its peculiar shape. Since the front refracting surface causes an extremely uneven reflection due to its undulation, the lens becomes unsightly.
A lens having its back refractive surface made aspherical, is known from Japanese Laid Open Patent Publication No. 84741, Japanese Laid Open Patent Publication No. 84742/78, Japanese Laid Open Patent Publication No. 195826/83 (IT 48315/82) and Japanese Laid Open Patent Publication No. 60724/85. These lenses suffer from the problems that their aspherical back refractive surfaces are not, aesthetically appealing when used in eyeglasses because their front refracting surfaces are made as a convex toric surface or a cylindrical surface in the case of an astigmatic lens. Further, since the eyeglass lens which is presently widely used has its back refractive surface formed as a concave toric surface utilizing a lens manufacturing machine adapted to manufacture that kind of lens, the manufacturing facility must be greatly modified when manufacturing the lens having the aspherical back refractive surface.
Accordingly, it is desired to provide a myopia corrective lens which overcomes the shortcomings of the prior art by providing a thinner lens having better overall optical efficiency.